While removing an electron from an atom forms a positive ion, that which has more than two electrons removed, such as, e.g., Xe44 + ion, is called a positive multicharged ion, which has an extremely large internal energy. It is known that bombarding multicharged ions on a solid surface causes many peculiar phenomena such as emission of a large number of secondary electrons and giving rise to a large structural change in nanometer size around a point of incidence of the multicharged ions (see References 1 and 2 in the list below). Unique interactions of such multicharged ions with a material have focused the spotlight of attention upon their feasible application to processes in a nanometer region such as single-ion implantation and fabrication of a quantum dot (see Reference 3 in the list below). As the ion source from which to produce such multicharged ions there are generally known electron cyclotron resonance (ECR) type ions generating source (ECRIS) and electron beam type ions generating source (EBIS), the latter being featured by high degree of ionization of ions obtained.
As an EBIS according to Prior Art 1 there is known an apparatus of the National Institute for Fusion Science that has been developed for researches in the atomic physics (see, e. g., Reference 4 in the list below). This apparatus comprises an electron source (cathode), a drift tube, a collector, a solenoid magnet and an ion extracting lens so configured that electrons exiting the cathode are passed through the drift tube and collected by the collector. The electrons are compressed by a strong magnetic field formed in the drift tube, becoming an electron beam of large current density. On the other hand, a gas introduced from the vicinity of the cathode is ionized within the drift tube. The drift tube is split into a plurality of electrode regions across both ends of which is applied a square well potential having a barrier to ions. The ions are trapped in a given time period within the square well potential and become multicharged ions as ionization by their collision with electrons proceeds. Of such multicharged ions, those whose kinetic energy is increased beyond the barrier by their collision with electrons are led towards the ion extracting lens so that they are taken out of the ions generating source.
Further, in 1988 EBIT (electron beam ion trap) according to Prior Art 2 was developed which was improved over the EBIS according to Prior Art 1 (see Reference 5 in the list below). The EBIT of Prior Art 2 which is identical in principle of generating multicharged ions to the EBIS uses a superconducting Helmholtz type coil and a reduced length of the drift tube such as to avoid the instability of plasma in the drift tube, thereby improving the confinement time for ions so that the high multivalent muticharged ions can stably be retained. As a consequence, in the EBIT it has been made possible to squeeze an electron beam in the drift tube to the ultimate to form highly ionized ions.
As an EBIT according to Prior Art 3 there has also been developed by the present inventors an apparatus (see Reference 6 in the list below) that has an electron accelerating voltage of 300 kV at its maximum to allow completely ionizing uranium (U). This EBIT was developed for researches in the atomic physics and has the highest performance in the world as the internal energy of multicharged ions that can be produced. There have been the problems, however, that the apparatus is expensive and costly in operation to obtain high beam intensity.
As Prior Art 4, there has also been developed a REBIT in the Lawrence Berkeley Institute of the USA where the superconducting magnet in the EBIT is cooled by a closed-cycle type refrigerator (see Reference 7 in the list below).
As the EBIT according to Prior Art 5, a commercial product using a permanent magnet was developed by Technical University Dresden jointly with Leybold Vacuum in Germany, but is weaker in beam intensity than the EBIT using the superconducting magnet since a permanent magnet is weak in magnetic field (see Reference 8 below).
As the EBIS of Prior Art 6, an apparatus of Brookhaven Laboratory in the USA (see Reference 9 in the list below) separates a superconducting magnet from an electron source, a drift tube and a collector that are required to produce ions (and which are herein referred to collectively as an ion source electrode). In separating the superconducting magnet from the ion source electrode, however, this apparatus entails breaking vacuum in the ion source electrode. As a result, the EBIS of Prior Art 6 due to weak heat resistance in a region of its superconducting magnet cannot have enough degree of vacuum obtained unless it is pumped continuously over an extremely long period of time. Further, while in the EBIS the electron source, drift tube and collector must have their central axes mutually aligned with due precision, considerable amounts of time are required in their assembling and alignment since with the superconducting magnet unassembled it is not possible for the ion source electrode alone to be aligned axially.
Reference 1: J. W. McDonald, D. Schneider, M. W. Clark and D. DeWitt, Phys. Rev. Lett., Vol. 68 (1992), p. 2297;
Reference 2: T. Meguro et al., Appl. Phys. Lett., Vol. 79 (2001), p. 3866;
Reference 3: T. Schenkel et al., Appl. Phys., Vol. 94 (2003), p. 7017;
Reference 4: Nobuo Kobayashi, Shunsuke Oya and 7 others, Plasma Laboratory at Nagoya University, Data and Technical Report IPPJ-DT-84, 1981;
Reference 5: M. A. Levin and 7 others, Physica Scr., T22, 1988, p. 157;
Reference 6: Shunsuke Oya, Makoto Sakurai, Journal of Plasma and Nuclear Fusion Society, 73 (1997), p. 1063;
Reference 7: Physics & Technology, Inc., “Refrigerator Cooled Electron Beam Trap/Ion Source” (REBITS/S), [online], (publication date unclear), [Searched on Oct. 5, 2004], Internet <URL: http:/www.physicstechnology.com/pt brochure.pdf>;
Reference 8: G. Zschrnack, “The Dresden EBIT”, [online], (publication date unclear), Technical University Dresden, Atomic Physics Group (Searched on Oct. 5, 2004), Internet <URL: http:// www.physik.tu-dresden.de/apg/apeebitl.htm>; and
Reference 9: N. Beebe et al., Rev. Sci., Instrum., Vol. 73 (2002), p. 699.
The multicharged ions generating sources according to Prior Arts 1 to 5 entailed considerable mount of time and cost in its manufacture, adjustment, operation and maintenance, since their ion source electrode comprising an electron source, a drift tube and a collector and their superconducting magnet are disposed in a common vacuum chamber and their beam intensity is weak.
Further, there is the case as in the multicharged ions generating source according to Prior Art 6 where the superconducting magnet is separated from the ion source electrode required to produce ions. Then, the need arose to beak vacuum in the ion source electrode. As a result, considerable amounts of time are required in their assembling and alignment since with the superconducting magnet unassembled it is not possible for the ion source electrode alone to be aligned axially.
Thus, the multicharged ions generating sources according to Prior Arts that are all designed for researches in the atomic physics are defective in their operability and maintainability and moreover are weak in beam intensity. Thus, no practical source of generating multicharged ions has yet been implemented for use in machining to form, e.g., a nano structure (microfine device in a nanometer order).